The basic principles of refrigeration are derived from the behavior of fluid refrigerants when they change from a liquid to a gas, or from a gas to a liquid, and absorb or release latent heat to the environment. Such basic principles have long been utilized in ice making systems. Suitable fluid refrigerants include carbon dioxide and halogenated hydrocarbons.
An ice making system is typically made up of three principle components--a compressor, a condenser, and an evaporator--which generally comprise a closed system. A fourth component--an ice forming mold--is usually placed in close thermal contact with the evaporator.
During the freeze cycle, the compressor receives a vaporous refrigerant at low pressure and compresses it, thus increasing the temperature and pressure of the vaporous refrigerant. The compressor then supplies this high temperature, high pressure vaporous refrigerant to the condenser, where the refrigerant condenses, changing from a vapor to a liquid. In the process of condensing, the refrigerant releases heat to the condenser environment. In a large ice making system, where a considerable amount of heat is released, the condenser is usually located far from the compressor, and typically outdoors.
From the condenser, the liquid refrigerant is supplied to the evaporator, where the liquid refrigerant changes state to a vapor. In the process of evaporating, the refrigerant absorbs latent heat from the surrounding environment. From the evaporator, the refrigerant returns to the compressor. During the freeze cycle, the mold of the typical ice maker is cooled to well below freezing while water is pumped over the mold to build up the desired ice forms.
After the ice has formed, the typical ice maker goes into a harvest cycle in which hot gas from the compressor is fed directly to the evaporator to heat the mold and thus free the formed ice. Because heat from the vaporous refrigerant is used to free the formed ice, ensuring an adequate transfer of heat from the compressor to the evaporator during the harvest cycle is a significant concern, particularly in those situations as described below where an ice maker having a remote condenser is to operate over a range of ambient temperatures.
In many ice making systems which are designed to operate in low temperatures (e.g., below 50.degree. F. ambient), a head pressure control valve is provided, in part, to maintain a minimum head pressure to ensure that compressor heat will be available for the next ice harvest cycle. Such a head pressure control valve is generally designed to back-up liquid refrigerant in the condenser during cold temperatures. This back-up procedure, however, results in extra refrigerant charge being added to the system. As long as the system has enough receiver capacity, this extra refrigerant does not hurt the system during the freeze cycle as the outdoor temperature rises. However, when the ice maker shifts to the harvest cycle, this extra refrigerant can overload the compressor and damage the system.
During the harvest cycle, the vaporous refrigerant is supplied to the evaporator through a hot gas valve which typically has a fixed orifice that acts as a metering device. In self-contained systems with relatively small refrigerant charges, this works satisfactorily and provides acceptable harvest times without returning unacceptable amounts of liquid refrigerant to the compressor. However, in systems having large refrigerant charges (typically where the condenser is remote from the compressor), the discharge pressure during the harvest cycle tends to be much higher at elevated ambient (outdoor) temperatures than the discharge pressure in self-contained systems. This higher pressure causes more refrigerant to flow through the fixed orifice in the hot gas valve and into the cold evaporator where it condenses. If this condensed refrigerant subsequently reaches the compressor, the compressor can become slugged with liquid refrigerant and its efficiency can be materially impaired. The liquid refrigerant can also dissolve the lubricant in the compressor, and wash out the oil in the compressor, resulting in harmful friction between its moving parts. Reducing the orifice size of the hot gas valve, however, can cause unacceptably long harvest cycles when the discharge pressures are lower, such as occur at low outdoor temperatures.
In a preferred ice making system, a predetermined amount of refrigerant is circulated between the compressor and the evaporator during the harvest cycle. The amount of refrigerant to be circulated would vary from system to system depending upon operating conditions, such as the size of the evaporator. In one such arrangement, disclosed in U.S. patent application Ser. No. 852,523 assigned to The Manitowoc Company, Inc., the amount of refrigerant needed to ensure an efficient harvest cycle is monitored by the compressor's suction pressure, and additional refrigerant from the condenser is added if needed. This arrangement couples a normally closed solenoid with a harvest pressure regulating valve. The normally closed solenoid is open during the harvest cycle to allow vaporous refrigerant to flow to the regulating valve, which is preset to permit refrigerant to flow therethrough only so long as the suction pressure is below a predetermined level.